Extensible languages

Some mention should be made of "extensible" languages,
a trend that seems to have reached its height with two symposia circa 1970,
[Chri 69,
Schu 71].
The general idea was that the programmer would be given the means to construct
a specialized language for any arbitrary application area by extending a base
language. In practice, the name "extensible languages"
was applied to everything from SIMULA, to PL/I
(because of its bewildering macro facility), to compiler-compilers.
The trend lost momentum after the symposia; see
[Stan 75].

Although the concept of extensible languages appears quite similar to the
adaptivity principle, very little of the research done was accompanied by
formal grammar models. An exception is Wegbreit's
ECFGs;
W-grammars might also be excepted,
since ALGOL 68 was painted as an
extensible language in the earlier of the two symposia.
Also, a pathological case was the extensible language
AEPL,
in which the language processor was driven by a dynamically
modifiable set of Chomsky type 0 grammar rules with programs attached to them.